On the Fissure-Inclusions. 109 



these fluid inclusions, upon the stage of the microscope, 

 changes invariably occur in the form, size, position, or 

 velocity of motion of any bubble under observation, and 

 in the volume of the liquid in which it floats. In .fact, 

 the relative changes in size of this drop at different tempera- 

 tures, and the complete occupation of the cavity by expansion 

 of the liquid, with a disappearance of the bubble at an observed 

 temperature, sometimes afford an accurate and ready standard 

 for the determination of the coefficient of expansion, etc., of 

 certain liquids found in these cavities, and thus of their iden- 

 tity. For example, the temperature at which liquid carbonic 

 acid assumes this condition, reaches its "critical point" and fills 

 the cavity, is about 30-32° C, while with inclusions of water, 

 or of saline solutions, which are by far more common, the 

 section may be heated up to the point (about 150° C.) at which 

 the hardened balsam, in which it is mounted, begins to soften, 

 without any material change being produced in the relative 

 sizes of the bubble and liquid. Sorby, in his well-known pa- 

 per,* and after him, J. Clifton Ward, f have not only deduced 

 from the presence or absence of cavities, and from the charac- 

 ter of their contents — fluid, glass, or stone — whether the crys- 

 tals or rocks in which they occur have been formed from aque- 

 ous solution, igneous fusion, or both conjoined ; but they have 

 availed themselves of the relative volumes of the liquid and 

 bubble in carefully selected cavities to determine approximately 

 the genetic conditions of their formation, temperature, 

 pressure, and rapidity of cooling. This relationship has de- 

 pended both on the temperature and on the pressure, usually 

 enormous, under which the rocks became solidified. The co- 

 efficient of expansion of the liquid being known, and a certain 

 probable temperature being assumed, at which the rocks in 

 question, granites and elvan, were plastic, the amount of pres- 

 sure was calculated with its equivalent in feet of superincum- 

 bent rock. This last amount was always found to be greater 

 by 15,000 to 20,000 feet, than that indicated by the stratigraph- 

 ical examination of the region, but this excess was attributed 

 to the later.-'.l pressure produced in the folding and crushing 

 •of the mass during its upheaval into mountain ranges. 



The only purpose in the cursory review just presented has 



*Quart. Journ. of Geol. Soc, 1858, XIV., 453. 

 \Idem, 1875, XXXI., 568, and 1876, XXXII., i. 



